| The Internet of things provides new thoughts and methods for the underground coal mine safety monitoring and disaster warning. By utilizing the theories and technologies of Internet of things, the underground coal mine Internet of things percepts the persons, the equipment, theenvironmental and etc underground, with the use of a variety of ubiquitous sensors, and realizes the monitoring and warning of several disasters. Many apphcations of the undei’ground coal mine Internet of Things perception layer, including node location, data fiisioA collaborative transmission,time division multiple accs technology, etc.,all need those nodes participated have the consentaneous time, thus time synchronization is a supporting technology for this perception layer. The particularities of the coal mine worfdng environment, such as the space is narrow, the number of sensor nodes is laiy,wirelsss communication environment is poor, make toe existing time synchronization methods for wireless sensor netwoiks above ground cannot be applied directly.Due to the demand on coal mine safety production and the pardcularity of the coal mine wireless environment,this paper studisstoe time synchronization fixtoe underground coal mine Internssof Things perception layer,and mainly aimssat the improve of its energy efficiency.Themain work and contributions are summarized as follows:1) Considering the long and narrow coal mine laneway underground, a hierarchical time synchronization algorithm is researched and the feedback mechanism is applied to the time synchronization, while toe synchronization cycle and duty cycle are regulated according to toe network synchronization precision error based on the principle of negative feedback.A tree-type hierarchical topology is established for the perception layer networic,and several parent nodes and pare-wise synchronization child nodes are selected for the network.The synchronization information are two-way exchanged between the parent node and the pair-wise synchronization nodes for many times,meanwhile those synchronization information are overheard by those overhearing nodes,thus the amount of synchronization information transmitted throughtoe network is reduced.The time synchronization mode fortoe inner-group nodes is chosed astoe periodic one,namely,the parent node launches one more synchronous cycle for the imer-group nodes whentoe synchronization cycle is due,whUetoe node for the inter-group nodesor fortoe remote node and the sink node are set as the on-demand one,that is to say,only when specific events occur wUItoe time synchronization betweentoem be launched,thustoe amount of synchronization infoimation transmitted through the network is further reduced.The clock offset between sensor nodes are estimated with the utilizing of the joint maximum likelihood estimation method,Wle the subscript offset method is used to reducetoe number of estimation parameters,therefore computational complexity of the clock offset estimation algorithm is somewhat simplified.The network synchronization error is periodically evaluated by the sink node,and on thebasis of negative feedback principle,the network synchronization cycle and duty cycle are regulated by the sink node with the use of proportional control law accordingtothe disagreement between the actual andtoe required synchronization error,thustoe synchronous cycle is enlarged and the duty ratio is reduced as much as posstole,intoe casetoattoenetwork,s synchronizationprecision is met with.2)To improve the time synchronization algorithm,s robustness to the network,s topology,theconsensus algorithm in control theory is applied to the time synchronization one for theunderground coal mine Internet of Things perception layer, and a kind of second-order consensus time synchronization algorithm is proposed, moreover the energy efficiency of the coherence timesynchronization method is improved from the network’s adjacency weight optimization design aspects.The node’s clock feature is modeled into a second-order state equation,and toe node’s statesare broadcasted to its neighboring nodes periodically according to the synchronization cycle, whiletoe control input is constructed according to the state disagreement between neighboring nodes.Conditions on the convei,ence of the proposed second-order consensus time synchronization algorithm, together with those factors that influence its convergence factor, are analyzed with the use of matrix transformation and Routh-Hurwitz stabUity theory. Factors the affect the time syncluonization algorkhm,s energy consumption are forther retoarched, and to is concluded that toe energy consumptiorfs optimization problem can be converted into toe weighted optimizationproblem of the convergence factor and the network,s sparsification problem, which can be reaHzed from toe design on network,s adjacency weight matrix. A distributed algorithm to dssign this adjacency weight matrix is proposed.3)From toe perspective of networic communication rate,toe energy efficiency of second-order consensus time synchronization algorithm is further considered,and a kind of event trigger second-order consensus time synchronization algorithm is proposed.The node’s communication event is set as the event-triggered mode, and a distributed event trigger function is designed for each node, thus only when the vaule of this trigger function is greater than zero will tiie node trigger a new broadcast communication event The convergence of the proposed event-triggered second order consaisus time synchronizadcMi algorithm is solved with the use of matrix transformation and Roufli-Hurwitz stability theory, meanwWle tiie distributed design for the tri,er threshold is presented Furthermore, the fectors affecting Ihe network communication rate are analyzed WAi the comparison on relative convergence with the time-triggered one,it is concluded that with Ihe assuiT5>don of the same networic communication rate, the event-trgger time synchronization algoriflim ouperfortns the one with time-triced communication. |
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